Wireless networks can determine the position of a mobile device based on trilateration using, for example, time difference measurements taken by the wireless handset, or by components installed in the wireless network or adjunct to it.
The challenge of trilateration on wireless signals, e.g., in PCS and cellular networks, is that those networks have transmitters and/or receivers that are either unsynchronized or loosely synchronized to a timing reference that is of insufficient accuracy to support location via trilateration. This problem can be corrected by calibrating the wireless network and/or Position Determination Device to determine these calibration values and in turn subtract their effects from subsequent position calculations.
Calibrating a network can be an expensive and time-consuming process. In order to achieve statistical significance, many calibration data samples are required. Traditional methods utilize a substantial amount of test equipment to obtain the necessary calibration data. A typical method requires the tester to generate a significant number of calls from a wireless handset. Each call is received by the Mobile Switching Center, which forwards the call to another device that accepts the call and issues one Position Request message per call, generating a single Position Response Only one calibration sample is generated per Position Response, therefore many calls are required to generate a sufficient number of samples. To generate the number of Position Requests required for statistical significance, the tester must make an equal number of calls, which is generally time consuming because the tester has no knowledge of when the Position Response was received by the device that issued the Position Request. The limiting factor is that this traditional method generates only one calibration sample per invocation of the Position Determination process when used for calibration purposes.
A calibration sample is defined herein as a measurement of the difference between an absolute time reference, and the actual time reference being used by the components in the wireless network. Each reference point in a wireless network can be associated with an observed time, and the calibration value associated with that reference point is subtracted from the observed time reference to achieve a reference closer to absolute time. The calibration value is comprised of many calibration samples via averaging or other statistical methods.